1. Field of the Invention
This invention relates to a substrate primer for bonding of polymeric surfaces with cyanoacrylate adhesives, and in particular, relates to trialkylammonium alkyl carboxylate primers.
2. Description of the Related Art
Cyanoacrylate adhesive compositions are widely used to bond substrate materials such as metals, wood, glass, ceramic and leather, due to their rapid fixturing properties and their high bonding strength, which for metal-to-metal bonding may be on the order of 1,000 to 3,000 psi as measured by the lap shear strength determination method of ASTM D-1002-64. It is extremely difficult, however, to bond substrates such as low surface energy, non-polar plastics, e.g., polyethylene and polypropylene, with these adhesives.
Although surface treatment methods, such as flame treatment, corona discharge treatment, and chlorochromate oxidizing treatment, have been used to improve cyanoacrylate adhesion to non-polar substrates, these treatment methods generally are complicated and expensive and do not result in uniform treatment of the surface.
A number of primer compositions have been developed for application to hard-to-bond surfaces such as non-polar substrates prior to application of the cyanoacrylate adhesive composition, so that the bonding efficacy of the cyanoacrylate adhesive is enhanced. U.S. Pat. No. 4,460,759 to J. Robins, issued July 17, 1984, discloses an adhesive composition comprising an adhesive base which includes an alphacyanoacrylate and a stabilizer, in combination with an accelerator compound of the formula MA, wherein M is a cation whose pK.sub.a value in aqueous medium is at least about 10, and A is an anion whose pK.sub.a value in deprotonation equilibrium reaction is less than or equal to about 0. The adhesive composition described in this patent is said to have a set time less than one third of that of the adhesive base alone and an overlap shear value of at least 400 lbs per square inch. Among the cation M species disclosed in the Robins patent are onium cations such as quaternary ammonium cations (see column 4, lines 42-49 of the patent). Illustrative anion A species are exemplified in the disclosure at column 5, lines 8-15 of the patent. The disclosures of this patent and all other references cited herein are hereby incorporated herein by reference.
Organometallic primers such as aluminum alcoholates and aluminum chelates have also been used with cyanoacrylates. European Patent Application No. 129,068 published Dec. 27, 1984, discloses a primer said to be useful for bonding nonpolar or highly crystalline substrates together, or for bonding another material, such as a solid material, coating material, paint or printing ink to the substrate, by an alphacyanoacrylate. As examples of such difficult-to-bond substrate materials, this publication mentions polyethylene, polypropylene, polybutene, polyfluoroethylene, and their copolymers, as well as other polymeric substrates such as polyethylene terephthalate, polyacetal, nylon and soft (highly plasticized) PVC film. The disclosed primer comprises one or more organometallic compounds, such as metal alcoholates, 1,3-dicarbonyl complexes, carboxylates, hydrocarbon group-containing metal compounds, and heteroatom-containing organometallic compounds.
Japanese Kokai No. 82/25,378 published Feb. 10, 1982, discloses solutions of rubber and isocyanates in organic solvents as primers for alpha-cyanoacrylate adhesives.
Kil Pikari, J., et al., Adv. Biomater. 1982, Vol. 4, pp. 101-105, "Adhesion Between Alkyl 2-Cyanoacrylates and Bone Pretreated with 1% Citric Acid," discloses the pretreatment of bone surfaces with 1% citric acid, tartaric acid, or succinic acid, to attempt to improve the adhesion of poly(alkyl 2-cyanoacrylate) thereto. These pretreatments are said not to improve the adhesion strength of the cyanoacrylate bone bond.
West German Offenlegungsschrift No. 2,950,523 published June 19, 1980, discloses a method of binding urethane formulations to rubber, by coating the rubber surface with a liquid cyanoacrylate adhesive, covering the wet adhesive layer with a polyurethane formulation, and hardening the polyurethane.
Japanese Kokai No. 74/12,094 published Mar. 22, 1974, describes the treatment of aluminum, polyester, and other substrates, with diethylamine, o-phenylenediamine, or a similar compound, to improve adhesion of such surface to poly(ethyl alpha-cyanoacrylate).
Japanese Kokai No. 78/35,744 published Apr. 3, 1978, describes the use of poly(methyl methacrylate) solution in dimethylketone as an adhesive composition for porous substrates, e.g., wood, for cyanoacrylate adhesive bonding thereof.
Primers for low surface energy substrates have also been prepared by dissolving chlorinated polyethylene or a chlorinated polypropylene in a solvent such as toluene. Examples of such primers are disclosed in West German Offenlegungsschrift No. 3,601,518, published July 24, 1986, and in PCT International Application Number WO 88/5346 Al, published July 28, 1988.
In addition to the above-described primer compositions for enhancing cyanoacrylate adhesive bonding of various substrates, there have been efforts in the art to incorporate additives into the cyanoacrylate bonding medium. These additives function as adhesion promoters for enhancing the bondability of substrates with cyanoacrylate adhesives. Such additives may, however, significantly increase fixturing time of the cyanoacrylate adhesive composition, thereby adversely affecting the utility of the composition in specific applications, or such additives may limit the utility of cyanoacrylate compositions containing same to bonding of only specific substrate materials, without effecting any improvement in the cyanoacrylate adhesive bonding of other substrates.
Co-pending United States application, Ser. No. 07/168,712, filed Mar.16, 1988, discloses use of tetraalkylammonium salts such as tetrabutylammonium acetate and tetrabutylammonium fluoride as cyanoacrylate primers for non-polar substrates.
The tetraalkylammonium salts, as well as many of the other primers disclosed in the above-discussed references, exhibit excellent adhesion promotion when the primer is applied and the specimen parts are bonded immediately. However, the primers disclosed above generally lose their ability to promote excellent adhesion when the primed substrate is exposed in air for a prolonged period of time. Since in many practical applications, the primed substrate is left exposed to air for a period of time before bonding, it is critical to maintain excellent adhesion promotion regardless of the amount of time the primed substrate is left in the open atmosphere.
Accordingly, it is an object of this invention to provide a primer composition that facilitates cyanoacrylate adhesive bonding of otherwise hard-to-bond substrates, such as nonpolar materials and other difficult-to-bond surfaces.
It is another object to provide a primer composition that does not deteriorate in performance upon prolonged exposure to air.
It is another object of the invention to provide a type of primer which can accommodate a variety of alkyl substituent groups in its chemical composition to variously enhance bonding to different surfaces.
Other objects and advantages of the invention will be more fully apparent from the following disclosure and appended claims.